The invention relates to a method for driving an open-end spinning rotor during automatic piecing, in which the spinning rotor is accelerated from a standstill to operating speed by being coupled to a tangential drive belt running at operating speed. The spinning rotor passes through a speed range suitable for a piecing, at which speed it is kept for a predetermined time span by changing the driving effect of the tangential drive belt. The invention relates further to an arrangement for carrying out this method.
In the case of a method of this kind (German published patent application 27 08 936 A1 and corresponding U.S. Pat. No. 4,172,357), control means are provided in a travelling piecing arrangement, which synchronize the devices for carrying out the piecing with each other and with the start-up phase of the previously resting spinning rotor in such a way that the piecing and drawing off of the yarn occurs during the acceleration curve of the spinning rotor. A piecing without reducing the revolutions of the entire spinning machine can be carried out in this way while still having an advantageous number of revolutions. In order to prolong the suitable speed range, the piecing device can intervene from the outside in the acceleration curve of the individual spinning rotor, for example by applying a brake. The rotational speed range best suited for piecing is thus kept constant for a predetermined time span.
It is also known (German published patent application 25 44 209 A1) that the current number of revolutions can be scanned in a non-contact way by a piecing device during the start-up phase of the spinning rotor, so that the start of the piecing program can be set in motion in dependence on the monitored starting-up behavior of the spinning rotor.
An object of the invention is to regulate the speed range of revolutions suitable for piecing in a method as mentioned above.
This object is achieved according to preferred embodiments of the invention in that the revolutions of the spinning rotor are measured and are close-loop controlled in dependence on the readings by means of reducing and increasing the driving effect of the tangential drive belt.
Contrary to the known piecing method, not only is the start-up phase of the spinning rotor prolonged and the piecing revolutions kept constant, but also the speed range suitable for piecing is closed-loop controlled for a predetermined time span. The actual number of revolutions can hereby more or less fluctuate around a set number. The test reading of the current rotor revolutions is thereby used not only to begin diverse piecing steps as in one known method, but also as a base for regulating the piecing revolutions.
In a development of the invention, a preset value of the rotational speed is given which lies in between two limiting values of the ideal speed range, to which the revolutions of the spinning rotor are matched; this is done by continuous reducing and increasing of the driving effect of the tangential drive belt. The driving of the spinning rotor is thus intermittently geared into from the outside. It is therefore advantageous according to preferred embodiments that the preset value of revolutions is maintained at least almost constant during the predetermined time span, which does not however rule out that the preset value of revolutions chosen for piecing cannot slowly increase during the predetermined time span. In a development of the invention, the driving effect of the tangential drive belt is reduced or increased by the intermittent activation of a rotor brake on the spinning rotor. The rotor brake already provided and used for bringing the spinning rotor to a standstill or considerably reduce the number of its revolutions, for example for a cleaning procedure before piecing, is used for this purpose. According then to this version of the invention, the rotor brake is repeatedly activated and released. The pressure of the tangential drive belt on the spinning rotor can, in conjunction with the rotor brake, also be changed intermittently, for example by means of repeated relieving and charging of the tension pulley.
In especially preferred embodiments for carrying out the method on an open-end rotor spinning machine, a travelling piecing device is provided, which travelling piecing device can be selectively placed at a spinning unit. The spinning unit comprises a spinning rotor driven by a tangential drive belt as well as an accompanying rotor brake. It is provided that a plotting arrangement and an accompanying servo component for proportional speed signals produced at the spinning unit are arranged at the piecing device, which servo component can be coupled with devices for activating the rotor brake. The piecing device thus uses components already positioned at every spinning unit to regulate the piecing revolutions.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.